Strange is not expecting to be yelled at. As such, he's in his own mind, thinking about how best to use his claw as a pen nib without breaking it or anything. So, as Peridot points her finger and yells at him, Strange does not expect that in the slightest.

As such, he tenses, tries to turn around in shock, but then just straight up falls out of his chair thanks to the combination of surprise and poor alcohol-ridden motor skills. Magician down! And turns out it's a confused magician down because all Strange can say to Peridot yelling at him is, "What?!"